This disclosure relates to fuel cell systems and specifically to sensing a concentration of fuel within a fuel cell system.
A fuel cell is an energy conversion device that generates electrical energy and thermal energy by electrochemically combining a gaseous fuel and an oxidant gas across an ion conducting electrolyte. Several types of fuel cells currently exist. A characteristic difference between distinct types of fuel cell is the type of material used for the electrolyte. The difference in the materials of the electrolyte employed distinguishes the fuel cells due to the operating temperature ranges of the materials. In one type of fuel cell, the Solid Oxide Fuel Cell (SOFC), the fuel cell is constructed from solid-state materials utilizing an ion-conducting oxide ceramic as the electrolyte. To generate a useful quantity of power, a fuel cell is made up of multiple fuel cell units in a series array, typically stacked together. A single SOFC unit consists of two electrodes, one is an anode and one is a cathode. The anode and the cathode are separated by the solid electrolyte just identified. Fuel for the SOFC is typically gaseous hydrogen and carbon monoxide supplied in from reformats, and the oxidant is commonly an air supply. The fuel cell operates when the oxidant contacts the cathode and the fuel contacts the anode. The electrolyte conducts the oxygen ions between the cathode and the anode maintaining an overall electrical charge balance in the system. Electrons are released from the fuel cell to an external circuit forming a flow of electrons. The flow of electrons released from the fuel cell to the external circuit provides useful electrical power.
The production of useful electrical power is the primary function of the SOFC. Optimizing the conversion of fuel in the fuel cell is an endeavor that commands a significant amount of time and effort. As in many other energy conversion devices, the function of converting the fuel into useful energy, (electrical energy, thermal energy), is closely monitored by system operators. Quantifying the concentration of fuel flowing in the fuel cell provides a benefit during the operation of the fuel cell. The performance of the fuel cell is related to, and optimized by knowing the concentration of fuel being supplied to the fuel cell. Understanding the fuel concentration allows operators to understand what quantity of fuel to supply, and what electrical load to apply. Unfortunately, directly measuring the concentration of fuel such as hydrogen in the fuel cell creates many engineering challenges due to the limitations of hydrogen concentration sensors. The limitations of directly measuring hydrogen concentrations with sensors are amplified when applied to the SOFC, because the SOFC operates at high temperatures and uses high concentrations of hydrogen. The limitations are greatest with respect to sensing the concentration of hydrogen and the material compatibility of the sensor.
Direct measurement hydrogen concentration sensors are designed for concentrations that are very small compared to the relatively high SOFC hydrogen concentrations that exist during fuel cell operation. As a result, the direct measurement hydrogen concentration sensors are inadequate for use with solid oxide fuel cells.
In addition to the forgoing, existing hydrogen concentration sensors that measure hydrogen concentrations directly are not compatible with SOFC operating environments. Typically SOFC""s exhibit high operating temperatures and a harsh environment both of which are detrimental to direct measurement hydrogen concentration sensors. Thus, there is a need in the art for a sensor that is compatible with both the operating environment and the relatively high levels of hydrogen concentration of the SOFC.
Fuel concentrations are determinable in a solid oxide fuel cell through voltage measurement of one or more fuel cell units, which voltage is a function of hydrogen gas present in the fuel feed stream to the one or more fuel cell units. The voltage in the one or more fuel cell units is proportionally related to the fuel concentration in the fuel feed stream to the entire fuel cell. A sensor determines concentrations of the fuel flowing in the fuel cell. The sensor comprises a fuel cell unit, and an indicator electrically coupled to the fuel cell unit, the indicator being capable of displaying a voltage or being adapted to convert a voltage to a fuel concentration display. The voltage measured is correlated to the fuel concentration flowing in the fuel cell. The above described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and claims.